signalk-bandg-hydra-nmea0183

signalk-bandg-hydra-nmea0183

Signal K plugin that feeds position and navigation data into the NMEA 0183 input of older B&G processors — the H2000 series (Hydra, Hercules) and similar hardware of that era — over TCP or UDP.

These processors accept NMEA 0183 input, but only an outdated form of it. They predate the current standard and reject or ignore sentences from a modern, compliant generator. This plugin produces what they actually parse: ddmm.mm coordinates (2 decimal minutes, as the H2000 manual specifies) with the NP talker ID. The non-compliance is the point — it is what makes the data usable to the processor.

So: if your kit accepts modern NMEA 0183, you want signalk-to-nmea0183 instead. Use this one when that output is too correct and the processor won't take it.

Sentences

Sentence	Contents	Sent when
RMC	Position, UTC time and date, SOG, COG, magnetic variation	Always (can be disabled)
RMB	Active waypoint navigation — XTE, range, bearing, VMG	A destination is set
APA	Autopilot format A — XTE, steer direction, arrival flags, track bearing	A course is active
XTE	Cross-track error and steer direction	A course is active

RMC already carries SOG and COG, so VTG is not sent. Each sentence can be turned off individually.

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Installation

Install from the Signal K App Store in the admin UI, then enable it in Server → Plugin Config.

Or from the command line:

cd ~/.signalk
npm install signalk-bandg-hydra-nmea0183
sudo systemctl restart signalk

Requires Node 18 or later.

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Setup

Everything is configured in the plugin itself — set the transport, destination address and port, enable it, and it starts sending. No Data Connection is required.

The plugin manages its own socket deliberately. Routing the sentences through a Signal K Data Connection was tried and abandoned: the admin UI only exposes a TCP output for it, and it applies validation that has to be disabled by hand — awkward for sentences that are intentionally non-compliant, which is the entire point here.

TCP or UDP?

	TCP	UDP
Connection	Client connection out to the instrument	Connectionless datagrams
Delivery	Retried and reported; status line shows link state	Fire-and-forget, unconfirmed
Reconnects	Automatically, every 5 s	N/A
Broadcast	No	Yes — e.g. 192.168.0.255

Prefer TCP when the instrument accepts a connection: you get real delivery feedback and the status line tells you when the link is down. Use UDP for devices that only listen, or to reach several at once by broadcast — accepting that nothing will tell you if data isn't arriving.

Because the plugin writes to its own socket, these sentences reach only the destination you configure. They never enter the server's shared NMEA 0183 output (port 10110), so nothing else on the boat sees them.

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Configuration

Option	Default	Description
Transport	TCP	TCP opens a client connection to the destination; UDP sends datagrams
Destination address	192.168.0.2	Host to send to. For UDP this may be a broadcast address
Destination port	1183	Destination port
TCP connect timeout (s)	5	TCP only. Stops a destination that silently drops packets from stalling for the OS timeout (~2 min) before retrying
Transmit rate (Hz)	1	How often sentences are sent
Maximum position age (s)	10	Past this age the fix is treated as stale and sentences go out flagged invalid (status V). 0 disables the check
Destination waypoint label	WPT	Fills the 4-character waypoint identifier field in RMB and APA. Clear it to leave the field empty
Send RMC	true	Position, SOG, COG, date, variation
Send RMB	true	Active waypoint navigation
Send APA	true	Autopilot format A
Send XTE	true	Cross-track error
Arrival radius (m)	100	Distance at which RMB/APA report arrival (the v2 arrival circle isn't in the v1 data model, so it's set here)

The plugin maintains one outbound connection and reconnects every 5 s if it drops or the destination is unavailable.

Reading the status

The status line updates every cycle, leading with the link state:

Active →tcp 192.168.0.2:1183 @ 1Hz | pos✓ cog✗ sog✓ var✓ rmc✓ wpt✓

If the destination can't be reached it turns red, carries the reason, and keeps retrying:

No link to tcp 192.168.0.2:1183 (error: connect ECONNREFUSED) — retrying every 5s, sentences discarded

ECONNREFUSED means the host is up but nothing is listening on that port; connect timeout means the host isn't answering at all. The pos/cog/sog/var ticks report Signal K input availability — they say nothing about the link, so read the link state first.

On UDP there is no link state. The status shows Active →udp … whenever the socket is open, which it essentially always is. UDP cannot tell you whether anything received the data; use TCP if you need to know the instrument is really being fed.

Enable debug logging in the admin UI to see each sentence as it goes out:

$NPRMC,071142,A,3352.08,S,15114.03,E,6.1,045,160626,12,E*5F
$NPRMB,A,0.02,L,,WPT,3352.00,S,15113.00,E,1.2,048,5.9,V*6E
$NPAPA,A,A,0.02,L,N,V,V,048,M,WPT*72
$NPXTE,A,A,0.02,L,N*65

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How it works

1. On a timer at the configured rate, the latest navigation.position is read from the Signal K data model.
2. RMC is built (decimal degrees → ddmm.mm + hemisphere) with UTC time and date, plus SOG, COG and magnetic variation when available. The RMC COG field is true-referenced; if navigation.courseOverGroundTrue is absent it is derived from navigation.courseOverGroundMagnetic + navigation.magneticVariation.
3. If a course is active, the enabled course sentences are built from navigation.course.calcValues.*: RMB (cross-track error, distance, bearing, VMG — the waypoint position is recovered from vessel position + bearing + distance via great-circle forward), APA (XTE, steer, arrival flags, track bearing) and XTE.
4. Each checksummed, CRLF-terminated sentence is written to the TCP connection or sent as a UDP datagram.

The output format follows the legacy legacy_gps.py processor.

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Notes and known limitations

Course data comes from the v1 model. app.getSelfPath reads the v1 data model, where navigation.course exposes only calcValues.* — the v2 Course API's nextPoint, previousPoint and arrivalCircle are not visible there. So an active course is detected via calcValues.distance, the waypoint lat/lon is derived rather than read directly, and the arrival radius is a plugin option instead of being taken from the arrival circle.

Precision is deliberately coarse, in two places. Lat/lon are emitted as ddmm.mm (2 decimal minutes, ~18 m) and bearings as bare integer degrees (045, not 45.2) — both to match the field widths in the Hydra 2000 manual exactly. The manual gives COG, RMB bearing, APA track and magnetic variation no decimal place at all, so 1° is the available resolution for those. To revisit: if your instrument accepts extended precision, ddmm() and deg3() in index.js are the two places to widen.

Stale positions are flagged, not hidden. Signal K serves the last known navigation.position indefinitely — if the GPS loses lock or its source dies, the data model keeps returning a frozen fix with nothing to mark it old. Sending that with status A would have the processor navigate on it. So the position's timestamp is checked each cycle, and once it exceeds Maximum position age the RMC, RMB, APA and XTE sentences are still sent but with status V, which tells the processor to discard them. The status line turns red and reports the age. Sources that publish no timestamp can't be judged and are treated as current.

Check the steer direction against your pilot. The RMB and APA steer direction (L/R) follows the Signal K convention that a negative crossTrackError puts the vessel left of track — so positive means steer left. That matches the spec, but confirm it against live data before trusting it to steer: it's a one-line sign flip in index.js if your pilot disagrees.

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Development

git clone https://github.com/ghotihook/signalk-bandg-hydra-nmea0183 ~/signalk-bandg-hydra-nmea0183
cd ~/.signalk && npm install ~/signalk-bandg-hydra-nmea0183
sudo systemctl restart signalk

# after changes
git -C ~/signalk-bandg-hydra-nmea0183 pull && sudo systemctl restart signalk

Tests

npm test

No dependencies — the suite runs on the built-in Node test runner. It covers the field formatting (where every bug found so far has been) and drives the plugin end to end against a throwaway TCP listener, checking the emitted sentences against the field diagrams in the Hydra 2000 manual.

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License

Apache-2.0